Five Great Patterns for Dealing with References

n a UML class model, classes are described by having attributes, operations, and relationships with other classes. When such a model is converted into code, both the public attributes and the relationships will appear as properties in your classes. These properties have types, which generally fall into one of the following categories:

• Basic types. Simple properties such as FirstName, StartDate, or Limit have simple property types, such as string, datetime, or int.
• Value types. Properties such as Email, ZIP, or CreditCard are a bit more sophisticated. Such properties tend to enforce validations on the possible values of properties, including regular expression and range checks. Therefore, types of such properties are described in value types, such as BankAccount, ZIP, or CreditCard.
• Business class. Properties of classes that implement a relationship with another class in general have the type of that class, either representing a single instance or a collection of instances of the related class. A Project class, for instance, is likely to have a property Manager of type Employee.
• Reference types. When the value of a property comes from a limited collection of values, the type of that property uses a reference type. There are several implementations for reference types, ranging from enumerated types, such as CourseLevel in a Course class to small referential classes, including a Country or Currencies class.

At first glimpse, enumerated types and referential classes appear to be to the most obvious patterns for implementing reference types, but there are other patterns that can do the job for you. This article includes five such patterns; I'll show you when each of these patterns is applicable and useful.